1. Field of the Invention
The present invention relates to an encryption device and editing device for a transport stream including moving picture experts group (MPEG) signal and a method for use therein.
2. Description of the Related Art
Recently, with the advance of digital signal processing technology, video signals are getting transmitted and stored in a form of digital signals. For transmitting and storing digital signals, an MPEG system is standardized.
FIG. 1 illustrates a conventional example of a processing system for MPEG signals on a personal computer basis.
Referring to FIG. 1, this system comprises a CPU 901, a hard disk drive 902, a digital broadcast processing board 903, a general bus 904, and a display unit 905.
The CPU 901 performs various kinds of arithmetic processing according to a program. The hard disk drive 902 stores various programs and data and particularly stores an encrypted transport stream. The digital broadcast processing board 903 performs various kinds of processing of a transport stream received from a digital broadcasting service. The general bus 904 is connected to the CPU 901, the hard disk drive 902, and the digital broadcast processing board 903, so that it transmits data to be input or output between them. The display unit 905 displays various screens, inputs video signals particularly output from the digital broadcast processing board 903, and displays a video according to the video signals.
The digital broadcast processing board 903 comprises a receiving unit 911, an encrypting unit 912, a decrypting unit 913, and an MPEG decoder 914.
The receiving unit 911 receives a transport stream from a broadcasting service. The encrypting unit 912 encrypts the transport stream received by the receiving unit 911 and outputs the encrypted transport stream to the general bus 904. The hard disk drive 902 stores the encrypted transport stream. The hard disk drive 902 outputs the encrypted transport stream stored therein. The decrypting unit 913 decrypts the encrypted transport stream and outputs an MPEG bit stream generated by demultiplexing video parts from the decrypted transport stream to the MPEG decoder 914.
The MPEG decoder 914 decodes the MPEG bit stream and outputs video signals obtained thereby to the display unit 905.
According to this arrangement, the transport stream appearing on the general bus 904 has already been encrypted. Therefore, it is impossible to copy the transport stream that has appeared on the general bus 904 illegally. In addition, the video signals output from the MPEG decoder 914 are directly output to the display unit 905, and therefore it cannot be intercepted. Thus, the copyright can be protected.
The prior art documents related to the present invention are as follows:    Japanese Laid-Open Patent Publication (Kokai) No. Hei 08(1996)-322034    Japanese Laid-Open Patent Publication (Kokai) No. Hei 10(1998)-336624    Japanese Laid-Open Patent Publication (Kokai) No. 2002-287624    Japanese Laid-Open Patent Publication (Kokai) No. 2002-287625    Japanese Laid-Open Patent Publication (Kokai) No. 2002-290999.
If the system as shown in FIG. 1 is used, however, a transport stream is encrypted completely by the encrypting unit 912, and therefore we cannot know what and where video pictures exist in the transport stream unless it is decrypted using the decrypting unit 913. Therefore, its content cannot be edited unless it is decrypted using the decrypting unit 913.
More specifically, if it is assumed that the transport stream decrypted by the decrypting unit 913 is edited by the editing unit 906 indicated by a dashed line in FIG. 1, data read from the hard disk drive 902 is decrypted by the decrypting unit 913, processed by the editing unit 906, encrypted by the encrypting unit 912, and then rewritten into the hard disk drive 902. Therefore, the decrypted transport stream appears on the general bus 904 as indicated by a dotted arrow line in FIG. 1. Thereby, the copyright cannot be protected.
Otherwise, there can be a situation that the encrypting unit 912 obtains representative images of GOPs from the transport stream not encrypted yet and edits the transport stream while viewing the representative images. However, even though a user desires to include the GOPs into the transport stream after editing, it is unknown where the GOPs exist in the transport stream. To clarify it, the encrypted transport stream eventually need be decrypted. Therefore, the copyright cannot be protected.